1. Field of the Invention
The present invention pertains to sputtering by ion bombardment, deposition of multicomponent films, such as alloy or compound films consisting of two or more elements from the periodic table, and the substrate-target geometry in sputter deposition of a coating. More particularly, the present invention concerns a method for sputter deposition of films or coatings over large areas or at widely different substrate locations relative to the target where the coating is of exactly the same solid component composition as the target from where the material is being sputtered.
2. Description of the Prior Art
The main reason for deviations in a coating composition from that of a flat target is that the various species sputtered from the target have different angular distributions. If one species is for instance preferentially ejected in a direction normal to the target surface, then the law of CONSERVATION OF MASS in a closed system requires that this species must be deficient at another angle of ejection. Integrating the species over all ejection angles from zero (normal ejection) to +/-90 degrees must necessarily add up to the exact target composition. However, this conservation of matter does not apply for volatile gases such as oxygen when they are partly lost into the pump or gettered in other parts of the apparatus.
The angular distribution of the constituents in binary alloys was first studied by the inventor and published in a paper: R. R. Olson and G. K. Wehner, Journal of Vacuum Science and Technology, Vol. 14, page 316 (1976), followed by a more detailed later paper; R. R. Olson, M. E. King, and G. K. Wehner in Journal of Applied Physics, Vol. 50, page 3677 (1979). The sputtering of multicomponent materials is described in a survey article by Betz and Wehner in Topics of Applied Physics, Volume 52, Springer Verlag, N.Y. 1983.
The target geometry played a role in the early attempts in magnetron sputtering where cylindrical, conical and hemispherical targets (in the latter case for sputtering towards the inside of a hemisphere onto a flat substrate) were considered before magnetron sputtering from a planar target was invented, and patented by Chapin in U.S. Pat. No. 4,166,018.
If a flat planar alloy target contains only two metals, one can always find an ejection angle in which the composition is the same as in the target. But this is not usually the case when more than two elements are involved because each species has its own angular distribution and there exists no ejection angle in which all three would come together to form a film with exactly the composition as that of the target. Furthermore, to be restricted in substrate location relative to the target is highly impractical. Another complication with a planar target is the fact that the angular distributions of different species change with the bombarding ion energy.
The present invention overcomes the disadvantages of the prior art by not sputtering from a flat, but from at least one spherical or partially spherical target positioned in a uniform plasma of a triode or diode gas- or vapor-discharge or in a plasma which is created with radio frequency- or microwave-excitation.